Loaded transmission circuits



.Fuy 22. 1924. 1,501,959

w. H. MARTIN ET As..

LOADED TRANSMISS ION CIRCUITS Original Filed June 50, 1922 4Sheets-Sheet l IN V EN TORS g Mmmm' .uy 22 1924. LSLQSQ w. H. MARTIN ETAi..

LOADED TRANSMISS ION CIRCUITS Original Filed June 30, 1923 4Sheets-Sheet 2 By Z July 22, 1924. 1,50L959 w. H. MARTIN ET Ai.

LOADED TRANSMISSION CIRCUITS Original Filed June 30, 1922 4 Sheets-Sheet5 A TT ORNE Y uy 22 1924. LSLQSQ W. H. MARTIN ET AL LoADED'TRANsMrssIoNCIRCUITS 4 Sheets-Sheet 4 Original Filed June 30 1922 Kw @WMM QCM wwwATTORNEY the presence of cable Patented july 22, i324,

WILLIAM n. MARTIN, or New Yo, N. Y., aan 'momes smiw, or nacxnnseen, newJERSEY, nssrenorts To AMERICAN riannrnonr.v AND raanenarn cciaienny,

A CORPORATION 0F NEW YORK.

LOADED TRANSMISSION CIRCUITS.

Application led .Tune 30, 1922', Serial No. 571,898. Renewed November22, 1923.

To all fwlzom it may concern.'

Be it known that we, WILLIAM H. MARTIN and THOMAS SHAW, residing at NewYork and Hackensack, in the counties of Bronx and Bergen and States ofNew York and New Jersey, respectively, have invented certainImprovements in Loaded Transmission Circuits, of which the following isa specification.

This invention relates to the loading of transmission circuits, and moreparticularly to the loading of transmission circuits which are to beemployed for the transmission of carrier frequencies.

In multiplex systems employing carrier frequencies for transmissionpurposes, it has been found that open-wire lines are best adapted fortransmission of carrier frequencies, and, in general, cable circuits ofconsiderable length cannot -be employed for this purpose, because thecapacit-y and resistance. of the cable tends to greatly increase theattenuation of the higher frequencies. In the practical layout of aplant employing carrier transmission, however, the use of some cablelengths 'in a given circuit cannot Well be avoided by reason of the factthat cable is used in entering practically all large cities and is usedfor terminal purposes.

The resistance, inductance, capacity and conductance. respectively, of acable, are inherently different from those of open-wire line. Inconsequence of these differences, in an open-wire line circuit causesimpedance irregularities at the junction 'points of the cable and theopen-wire line.. These irregularities are transmitted to the telephonerepeaters l'and cause reduction in the gain of the repeaters. Theseirregularities also cause reflection losses and transmission distortion.

The impedance irregularity may be approximately eliminated by loadinglthe inserted cable so that it has as nearly as possible the samecharacteristic impedance. as the open-wire line with which it isassociated. In doing this the loading coil inductance and the spacing ofthe coils should be so chosen that the loaded cable circuits willtransmit eiliciently the different carrier fre-v quencies which aretransmitted over the open-wlre line.

to obtain three telephone circuits from each group of four wires-byusing each of two pairs of wires asa side circuit'and by usf` ing thetwo pairs together as a phantom circuit. In the application of carriertransmission to telephone circuits of this type, however, it is notexpedient to use phantom circuits for carrier telephone and telegraphtransmission systems because of crosstalk difficulties which would beencountered. Carrier systems are only superposed on physical circuits(that is, pairs of wires which are not phantomed) or on the sidecircuits of phantom groups. Consequently, where, as is usually the case,a phantom group includes cable in the open-wire circuit, it is desirableto load the side circuits to a cut-ofiI frequency sufiiciently high totransmit the carrier frequencies ordinarily .matter to load thesecircuits, if the side circuits and the phantom circuit of a group wereall loaded alike, the loading of the phantom for one cutl-oti' frequencyand the side circuits for a higher cut-off frequency involvesconsiderable difficulty. The expe-v dient of loading the phantom to thesame cut-off frequency as the side circuits is not practicable becausethe number of loading coils per unit length increases with quency of thecut-od` frequency, and to load the phantom with the additional coilsnecessary to bring it up to the same cut- 'ofi' point as the sidecircuits would be eX- tremely expensive when it is considered that noadditional transmission channels would be obtained from the phantom bythe extra loading.

Carrier systems as now operated require efficient transmission offrequenciesv up lto approximately 30,000' cycles. In order to obtainsatisfactory transmission and impedance characteristics in lthe entranceand intermediate cables through which carrier systems are routed, it is'necessary to space the loading coils at a relatively close spacing ofapproximately 900 feet. 0n the other hand, in connection with thetransmission of ordinary speech through cables inserted the fre.-

los

inl open-wire lines, Very satisfactory results are obtained by spacingloadingcoils at intervals of about 5,600 feet.

ln View of the fundamentally differenttransmission requirements ofcarrier circuits superposed on the sides of phantoms and the ordinaryspeech channel over the phantom circuit itself, it is convenient andeconomical to load each transmission circuit system with independentsets of coils spaced at approximately the intervals above mentioned. lnpractice, it has been found that six loading coils inserted in thecarrier side circuit for each loading coil inserted in the voicefrequency phantom satisfies the transmission requirements yabovereferred to and at the same time conforms to the exigencies of theordinary plant practice.

rlhe use of two different sets of coils for the carrier circuit and thephantom circuit loading involves certain difficulties in that.

the phantom circuit loading coils add certain effects to the carriercircuits which should be properly allowed for in the design of theloading, in order to obtain satisfactory transmission and impedance`characteristics in the carrier circuits. Correspondingly, the e'ects ofthe carrier circuit coils on the phantom circuit should be allowed forin the phantom circuit. loading design.

lit is one of the objects of the invention to provide a loadin unit tobe employed at the phantom loa ing point of a system in which the sidecircuits are loaded for car.- rier frequencies and the phantom forordinary voice frequencies, the loading unit being so designed as toprovide the proper loading e'ect both for the phantom and for the sidecircuits.

'llhe manner in which this object, as well as other objects of the`invention, is attained may be understood from the following detaileddescription of the invention, when read in connection with theaccompanying drawings, in which Figure l is a schematic layout of theloading of a section of cable intervening between the terminal' officeVand a phantom group of an open-wire line; Fig. 2 is a diagram showingthe layer arrangement of the windings of an ordinary phantom loadingcoil; Fig. 3 is a diagram showingthe capacity distribution of thewindings of the phantom loading coil of Fig.` 2; Fig. l is a diagramshowing the magnetic relation of the'windings of the phantom coil ofFig. 2; Fig. 5 is a diagram showing the layer formation of the windingslof the phantom coil of the loading unit employed in connection with thepresent invention; Fig. 6 is a diagram showing the capacity distributionof the phantom 'coil of Fig. 5;- Fig. 7 is a diagram showing themagnetic relation of the windings of the phantom coil of Fig. 5; Fig. 8is a diagram showing the layer formation of the windings of a sidecircuit coil to be employed in the loading unit of the presentinvention; Fig. 9 is a diagram showing the capacity distrlbution of thecoil of Fig. 8; Fig. l0 is a diagram showing the ma relation of thewindings of the coil o Fig. 8; Fig. 1l is a diagram showing themagneticv relation of the complete loading .unit of the presentinvention, and Fig. 12 is a diagram showing the capacity distribution ofthe loading unit shown in Fig. 1l.

As has already been described and as is illustrated in the drawing, thephantom loading coils lipx occur much less frequently than the carriercircuit loading coils Lc. lu the drawing, S1 and S, represent the sidecircuit drops at the terminal ofce, and S1 and S2 designate thecorresponding openwire connections' terminating at some point outside ofthe office. lin the circuits S1 and S2, phantom terminating transformersA, and A2 are inserted, and taps are taken from the midpoints of thecoils of these transformers/to form the phantom drop Px. linterveningbetween the terminal oiice and the terminals of the open-wire circuitsS1 and S2 are a number of loading points numbered from l to 6 inclusive.'llhese loading `points are preferably equally spaced, the spacingbetween two adjacent loading points being designated on the drawing asS. Side circuit coils Le are included in each side circuit at each ofthe loading points indicated, and in addition phantom coils LPX are included at the loading points numbered 2 and 5. rlhus it will be seenthat, as illustrated in the circuit shown, there is one phantom coil forevery three sidey circuit coils. rlhis number has been chosen in orderto simplify the illustration in the drawing, but it will be understoodthat in actual practice, where the carrier range extends to a frequencyin the neighborhood of 30,000 cycles, a greater number of side coilswill occur for each phantom. 'Six side circuit coils to each phantomcircuit coil have been found to be va satisfactory number, as hasalready been stated.

Considering first the reactance of phantom loading upon the carriercircuit loading installed on the side circuits, it will be obvious thatowing to the fact that the phantom loading coils occur much lessfrequently than the side circuit loading coils, the resistance,inductance and capacity eects which the phantom loading coils add to theside circuit tend to cause impedance irregularities. The conductancee'ects of the phantom coils upon the side circuits are negligible.

'llhe coil resistance edects may be minimized by designing the coils soas to obtain as low a resistance as is economically practicable inmeeting the other electrical requirements. ,'lhe inductance added to thecarrier circuit by the phantom coils due to etic apetece magneticleakage in the coils is, however, a somewhat more serious problem. Thismay be compensated for by making special adjustments in the carriercircuit .loading coil, which is installed at the phantom loading point.ln the design of the phantom coil, as will be pointed out later, thecapacity cfi'ects of the coil are ot' primary importance, and hence theleakage inductanceoi' the phantom coil must, to a large extent, bepermitted to attain whatever value is deter mined by capacityconsiderations. The can rier circuit loa-ding coil installed at thephantom loading point as noted above is designed to have an inductancesmaller than the inductance of the other side circuit coils by an amountequal to the inductance of the phantom coil in the side circuit. ln thisconnection it should be noted that general considerations of economy andplant lflea:l ibility make it desirable to install the phantom loadingcoil at the carrier loading points, so that the inductance can be takencare of in the manner stated.

As an example, suppose the loading ot the side circuit requires a sidecircuit co-il at each loading point having an inductance ot 5.25millienrys. Suppose, on the other hand, that the inductance ot' thephantom coil in the side circuit at the phantom loading points adds .l0milhcnry to the side circuit. lf it is necessary to bring the sidecircuit inductance to within one per cent of the iigure given` above fora side circuit coil, it will be apparent that the inductance added bythe phantom involves a greater variation in this section than will bepermissible. Accordingly, the inductance in this section may be takencare ot, so far as a side circuit is concerned, by providing` a coilJfor the side circuit at this loading point, having an inductance of5.15 milhenrys, so that the tot-al loading inductance in the side'circuit at this point will be 5.25 milhenrys.

The most troublesome irregularity from the standpoint of design is thatdue to the inherent mutual capacity between the side circuit linewindings of the phantom loading coil. As is well known, these windingsare interleaved in order to obtain good electrical balance. This balanceis necessary in order to avoid objectionable phantom-toside cross-talk.This construction causes an appreciable direc-t capacity between the twowindings. The windings also have a direct capacity to the case in whichthe coils are potted and a direct capacity to the loading coil core,which consists of iron.

The manner in which the windings of the usual type of phantom coil areinterleaved will be clear Jfrom the diagram of Fig. 2. In thisdiagram, 1. 2, 3 and 4 designate the four conductors of a quad in whichyconductors l and 2 are used for one side circuit and conductors 3 and 4for the other side circuit, the phantom being 'formed by conductors l'and 2 in parallel for current lnwing in one direction, and conductors 2iand 4 in parallel forming theV return conductor for current iowing inthe other direction. The core C of the phantom loading coil has fourquadrants, a, I), c and ci, and

two windings, an inner and an outer winding, are wound upon eachquadrant.

The windings wound upon the co-re are represented in the form ofsuccessive layers, beginning with a layer adjacent the core itself andextending to an outer layerl adjacent the pot P. F or example, the innerwindings ai has an inner layer adjacent the core quadrant a and itsouter layer is adjacent the innermost layer of the outer winding a0. Theoutermost layer of the winding ao is adjacent the pot P. The windings onthe other four core quadrants are similarly shown.

The interleaving of the windings will be apparent from the diagram. Aswill be seen, each winding included in a side circuit conductor isformed in two halves, one being an outer winding on one quadrant and theother half being van inner winding on the opposite quadrant.. Forexample, conductor 1 is connected to the outer layer ot' the winding a0upon the @quadrant of the core. The inner layer of this winding isconnected to the outer layer of the inner winding ci up-on the cquadrant of the core, and the innermost layer of the latter winding'isthen connected tot the conductor l. The other vconductor 2 of the sameside circuit is connected to the outer layer of the outer winding coupon the c quadrant of the core, an inner layer of said winding beingconnect/ed to the outer layer of the inner winding ai on the (L quadrantot'. the core, the inner layer of the latter winding being connected tothe conductor 2. The interleaving of the windings included in the sidecircuit conductors 8 and 4 will be apparent from the diagram withoutfurther description.

The principal capacities of the phantom loading coil arrangement arealso shown in Fig. 2. For example, there will be a capacity designatedas Kc between thel inner layer of each inner winding and the core C.There will, of course, be substantially no capacities between the otherlayers and the core, because the outerlayers are shielded by theinnermost layer. Similarly, there will be a capacity KW between theoutermost layer of each inner winding and the innermost layer of eachlouter winding. The capacity between other layers of inner` and outerwindings will likewise be negligible because of the shielding effect ofthe adjacent layers of the windings upon each quadrant. Finally, therewill be a capacity Kp between the outermost layer of each o-uter windingand the pot or casing P of the coil.

Of these capacities, capacity Kw is ordinarily largest as the adjacentlayers of each inner and outer coil lie directly upon each other,whereas the innermost layer of each inner Winding is separated from thecore by a fairly thick mass of insulation, and the outermost layer ofeach outer winding is likewise separated from the pot by a considerableair space.

rlhe electrical relations of the several windings and the distributionof these capacities are shown in Fig. 3. ln this iigure the windings areshown as resistances for the reason that, so Jfar as the side circuit isconcerned, the phantom windings, if ideally constructed, would have noinduotance and would, therefore, appear as pure resistance's. 'llhesegregation of the capacity near the inner or outer layer of a givenwinding is indicated schematically in the diagram by locating thecapacity-near the terminal convolutions of the resistance in thediagram.

lt will be obvious that the capacities above referred to produce aconsiderable capacity between the conductors of the side circuit at theloading point. llt has been proposed to allow for these capacity eiiectsof the .phantom loading coil upon the side circuit by treating thephantom coil as the equivalent (so ar as capacity is concerned) of abuilding-out condenser in the side circuit, which is used for carrierpurposes. .A s is well known, it is common practice, where a section isshorter than the normal loading section to shunt a capacity across theterminals of the section at the loading point, the capacitybeingdesigned to increase the capacity of the short section to that of anormal section.

While this expedient may be used`inl installations where the loadingsections are irregularl because not originally designed for carrierloading, it is desirable from plant considerations that loading sectionsbe made as far as possible of equal length in the original layout of theplant. Consequently, in n'ew cable installations involving carrierloading initially or ultimately, it is desirable, and usually will bepracticable, to lay out the cable man-holes to iit the theoreticalcarrier loading spacing, and it should not be necessary in a plant thusvdesigned to use building-out condensers to equalize the loading sectioncapacity. Under such conditions, the capacity introduced in the carriercircuits by the phantom loading coils constitutes a source ofobjectionable irregularity. ln long cables, the irregularity edects pile up at certain important carrier frequencies because of the uniformperiodicity of the recurrence of the individual irregularities. F orexample, in practical loading systems involving six side circuit loadingsections to each phantom loading section, and with the particular lengthof loading sections em- Leonesa carrier loading section which containsthe phantom loadin coil may be obtained by geographically sortening thisloading section. Neither of the above solutions is satisfactory ;trom aplant standpoint, as both solutions require special engineering andtesting work in the installation of the cables and interfere with theexibility of the use of the cable. lF rom transmission, engineerlng,plant and operating standpoint, it is desirable to have the carrierloading spacing uniform :tor all carrier loading sections including the*phantom loading sections, this being one of the important conditions forobtaining uniform section capacity values in the carrier loadedcircuits.

' lhe conditions just mentioned make it desirable to use at the phantomloading points combination phantom loading units consisting of a phantomcoil an two side circuit coils which have approximately the same totalresistance, inductance and capacity etfects in the side circuits as theregular carrier circuit loading coils which are installed at the loadingpoints where phantom loading is not required. lin accordance with thepresent invention, therefore, it is proposed to employ in the specialphantom loading unit a very low capacity type of phantom loading coilwhich is illustrated in the diagrams of Figs. 5, 6 and 7. llt is alsoproposed to combine with this low capacity type of phantom loading coilspecial carried loading coils of the type illustrated in lFigs. 8, 9 andl0, these coils having a lower capacity than regular carrier loadingcoils employed at non-phantom points by an amount which makes the totalside circuit capacity oi' the combination phantom carrier loading lunitapproximately equal to that of the regular carrier loading coils. llnthis combination u nit, the special carrier loading coils are adjustedto have an inductance which is lower than that of the regular carriercoils by an amount equal to the leakage -inductance which the phantomloading coil adds to the carrier circuits.

The reduction in capacity oit the phantom coil may be attained byproviding more insulation between the core and the innermost layer ofthe inner winding on each llO quadrant, thus reducing the'capacitydesig-` nated in Figs. 2, 3, 5 and 6 as Kc. More insulation and greaterseparation will alsoV be provided between the adjacent layers of theinner and outer windings, thereby reducing the capacity Kw which, asalready stated, is the principal capacity between the side circuitconductors introduced by the phantom loading coil. The capacity Kp mayalso be reduced by increasing the space between the outer layer of eachouter winding and the pot or casing. These expedients, of course,involve similar reduction of the winding space and hence require the useof smaller gauge conductors in the windings in order to get the desiredamount of inductance in the available winding space.

However, it is not suiiicient merely to have an equality of capacitybetween the special combination carrier-phantom loading units and theregular carrier circuit coils. In order to simulate the regular coilsfrom the capacity standpoint, it is desirable also that the capacitydistribution in the combination unit should be similar to thedistribution of capacities in the regular carrier circuit coils; thatis, the capacity distribution should be substantially symmetrical andthe largest direct capacities between the conductors of the side circuitshould be located near the effective line terminals of the unit. Theproblem then is to so design the phantom loading unit that the capacitybetween the side circuit conductors due to the phantom coil will bedistributed in substantially the same manner as the capacity in a sidecircuit coil. Fig. 8 shows the layer arrangement of the windings in aside circuit coil, and Fig. 9 shows the distribution of the capacities.The core of the side circuit coil where the side circuit is loaded forcarrier transmission will be oi' wood or other nonconducting material oflow permeability, so that there will be no capacity between the core andthe innermost layers of the inner windings. The capacities' CW betweenthe' adjacent layers of the inner and outer windings will becomparatively large and a smaller capacity Cn between the outer layersand the pot will also occur, as indicated in Fig. 8. As shown in Fig. 9,the capacities C... are effectively connected near the line terminals ofthe windings. The capacities to the pot are effectively connectedadjacent to the inner terminals of the windings A.,

and Rn. As a first approximation, they can be considered to constitutea` capacity shunt across the circuit, connecting the mid points of thetwo line windings. Fig. l() shows the magnetic relation of the windings.It will be observed that all of the windings are wound about the core inthe same direction and the connections are such that for currentsflowing serially over conductor 1 and back over conductor 2, the fluxdue to the coils will be aiding. This corresponds to the condition whencurrents are transmitted in talking over the side circuit. For currentstransmitted in parallel over the conductors 1 and'2, the inner and outerwindings upon each section of the core are opposing. This corresponds tothe condition when i minals thereof.

In order to obtain a capacity distribution in the side circuit of thephantom loading unit corresponding to that produced in the side circuitby the regular carrier loading coils, a capacity distribution such asshown in Fig. 6 shouldbe provided in the phantom coil. If we considerthe side conductors 'l and 2 in Fig. 6, it will be seen that thecapacities Kw, which correspond to the capacity Cw in Fig. 9, arebridged across the coils adjacent the line terminals thereof, and thesmaller capacities K,p are bridged across the coils between the midpoints of the windings in the same way as in Fig. 9. .The smallercapacities Kn between the core and inner layers of the windings are alsobridged across the coil circuit between points whichl are approximatelythe lmid points of the line windings. .The capacitiesvKc are thuseHectively in parallel with the capacities Kp. Consequently, we have a.condition, so far as capacity distribution is concerned, which veryclosely approximates that of Fig. 9.

Fig. 5 shows the layer formation of a phantom coil designed to obtainthe capacity distribution shown in Fig. 6. Comparing this diagram, withthat of Fig. 2, which illustrates the ordinary phantom coilcohstruction, it will be seen that in Fig. 5 the line conductor l isconnected to the innermost layer of the winding a.. instead of to theoutermost layer. Likewise, conductor l. is connected to the outermostlayer of the winding o, instead of to the innermost layer. The innermostlayer of the winding c, is in turn connected to the outermost layer ofthe winding a0. Similarly, the connections to the other windings aretransposed as illustrated, and without attempting to point out in detailthe Various transpositions in connections as compared with Fig. 2, itwill be suiiicient to state that line conductor 2 is connected to theoutermost layer of the winding o., instead of the outermost layer llO ofwinding co, and line conductor 2 is .con-

vnected to the innermost vlayer of winding co instead of to theinnermost layer of winding al. Conductor 3 is connected to the innerwinding b1 and conductor 4 to the innermost layer of winding al.,instead of the connections employed in Fig. 2.

The change in connections employed in Fig. 5 is necessitated by therequirements of the capacity distribution shown in Fig. 6.

of certain of the windings upon the core in order'to obtain the propermagnetic relations, as. shown in Fig. 7.'

Turning to Fig. 4, which shows the magnetic relations of the windings inthe usual Y type of phantom coil, it will be observed that all of thewindings, both yinner and outer,

A is reversed with respect to the other.

upon all four of the quadrantsare wound in the same direction but theconnections are such that currents flowing in parallel over conductors 1and 2 and returningover `conductors 3 and 4, will produce magneticfluxes aiding and in the same direction in all four quadrants. A currentflowing' in one direction, however, in conductor 1, for example, andreturning in conductor 2, will produce opposing luxes in the inner andouter windings of the a and 0 quadrants, so that if perfect magneticconditions obtain. there will be no inductance in the side circuit dueto thel phantom coil.

Comparing this arrangement with Fig. 7 which shows the. magneticrelations of the windings necessary to obtain the capacity relationsillustrated in Figs. 5 and '6, it will be observed that in Fig. 7 one ofthe windings on each quadrant` either inner or outr,

s shown, windings ao, bi, c, and d.; are wound in the same direction asin Fig. 4. Windings ai, bo, c0 and d, are wound in the oppositedirection to the. corresponding windings in 'Fig'. 4. Tf we consider thefluxes due to currents flowing in parallel over conductors 1 and 2 inone direction` and returning over conductors 3 and 4 in parallel, itwill be found that the fluxes due to all of the windings on all of thequarants are aiding and in the same direction. The fluxes produced bycurrents flowing over conductor l in one direction and returning overconductor 2, however.l will be opposing in the inner and outer windingsof a and c quadrants, so that we obtain the same result` magnetically inFig. 7 as in Fig. 4.

As has been previously stated, the phantom coil is to be combined withthe side circuit coils at the phantom loading point to produce thedesired inductance in the side circuit and obtain the desired capacityrela- Lacasse tions. Tn order to secure the desired symmetrical capacitydistribution in the loading unit comprising the phantom and two sidecoils, the side circuitloading coils are insented in circuit at theelectrical center of the line windings of the phantom loadingv coil, asillustrated in Fig. 12, which shows the resultant lcapacitydistribution. Fig. 11 shows kthe magnetic relations of the windings ofthe phantom and two side'coils when connected as shown in Fig. 12. Thisdiagram is self-explanatory and no further description is deemednecessary.

lin loading units such as is illustrated in and this involves a reversalof the directior-"Fig 11, it is also necessary 1n designing the coils toconsider the effects of the. carrier circuit loading coils at thevarious carrier loading points upon the phantom loading. lln thisconnection, however, the capacity eil'ect produced by the side circuitloading coils upon the phantom are of negligible importance as comparedwith the capacity et'- fects produced by the phantom loading coil uponthe side circuit. This is due to two reasons First, the ordinary phantomspeech frequency loading sections have approximately 10 times as much.mutual capacity as the ordinary carrier loading sections, thus being inpart due to the fact that an ordinary type of phantom circuit has amutual capacity per unit length which is approximately 60% greater thanthat of its own side circuits, and in part due to the approximate 6:1ratio between the physical lengths of the phantom and carrier circuitloading sections; second, the coil mutual capacity effective in thephantom circuit is inherently much less than the coil capacity which iseffective in the side circuits. This latter relation will be obviouswhen we consider that the principal capacity introduced` by the sidecircuit loading coils is between the two conductors of the side circuit,and substantially no capacity is involved between the windings of thecoil in one side circuit and the windings of the coil in another sidecircuit, for the two coils are entirely separate structures and will belocated physically a considerable distance apart, as compared with thedistance between the two windings of a given side circuit coil. Even thecapacity introduced by the phantom loading coil in the phantom itselfmay be disregarded, as in constructing the phantom coil, the windings inone side of the phantom circuit are wound upon diiierent quadrants ofthecore from the windings in the other side of the phantom, and thecapacity between the windings of the two sides ot the phantom will 'bequite small, vdue to the physical separation of the windings. Therefore,the capacity introduced into the phantom by the phantom loading coilitself is not ordinarily considered in the design of the loading coil.

llfil llt-r lil) lil)

Since the capacity may be disregarded as above stated, it will beapparent that the rincipal effectof the carrieror side circuit oadingcoils in the phantom circu1t 1s due to their resistance and leakageinductance. As the carrier or side circuit loading coils are usually ofthe air-core type (because of high frequencies employed in carriertransmission), the resistance and vinductance .effects are relativelylarger than those which occur in ordinary loading, where the iron coretype of loading coil is employed. The relatively large resistance ofthel air-core coils is a potential source of impedance 1rregularity atlow speech frequencies, Nhixe' the resistance of the conductor and thecoils is an im ortant factor in determining the characteristic impedanceof the loaded cable. If the ratio of resistance to inductance per unitlength in the loaded cable is differentfrom the corresponding ratio ofthe associated open-wire line, there will be a difference in impedancewhich will cause irregularity at the low speech frequencies transmittedover the phantom circuit. This effect will of course' also occur at thelow frequencies transmitted over the speech frequency channels of thecarrier circuits.

Tn order to prevent the impedance irregularities due to this dierence inratio from becoming objectionably large, it is desirable to design thecarrier loadin coils to definite resistance requirements, which dependupon the gauge of the cable conductor adopted as standard. No definiterule can be laid down, but in general the best practicable value ofresistance is determined by a study in which cost considerations arebalanced against transmission reactions. From the practical standpoint,the resistance requirements for the phantom loading units areestablished by the study which determines the resistance characteristicsof theregular carrier coils,

since ordinarily the latter coils occur six` times as frequently as thephantom loading units.

Coming now to the inductanceeffects upon the phantom circuit of thecarrier loading coils employed in the side circuit, it will be foundthat these inductance effects are greater than in the case of theiron-core coils which are used in the ordinary side circuit loading forordinary voice frequencies. This is due in part to the employment of theair-core type of coil, and in part to the design of the winding of thecoil. The aircore, being of very lowl permeability, producescomparatively large magnetic leakage, and this magnetic leakage producesa considerable inductance in the phantom, whereas, if the coil wereperfect magnetically, the side circuit coil would -produce no inductancein the phantom, owing to the balance of the windings. In designing thewindings of the coil, it is necessary to employ a conthe inductance inthe struction which will obtain a lovv mutual capacity. Thisnecessitates the use of a relatively large' amount of insulation betweenthe inner and outer section windings of the coil, with the consequencethat there is a greater separation between the windings. The increasedseparation, while reducing the mutual capacity, increases the leakageand results in a further inciease in phantom, due to the side circuitcoil..

In carrier loading coils of the type above described, the leakageinductance effect just considered amounts to nearly four per cent,Whereas with the ordinary 'type of loading coil, the leakage whichproduces inductance in the phantom from the side circuit coils is muchless than one per cent. This increased inductance effect in the phantomcircuit may be readily cared for where the side circuits are loaded forcarrier transmission, by considering the leakage inductance effects inthe phantom as distributed inductance. This y is entirely proper wherethe phantom is only used for the transmission of ordinary voicefrequencies because of the much closer spacing of the carrier sidecircuit loading coil. In other words, the phantom loading effect of theside circuit coils is electrically equivalent to continuous loading andcan be added directly to the distributed inductance of the phantom cablecircuit in designing the phantom coil loading.

As a practical example, it may be stated that the effects of carrierloading coils in an existing commerical circuit employing the principlesof the present invention supply approximately` fifteen per cent. of thetotal inductance per unit length, which is necessary for satisfactorytransmission over the cable phantom circuits. Therefore, the inductanceof the phantom loading coils required on the phantom circuitsA of thisinstallation, in which the side circuits are loaded for carrieroperation, is approximate- 1 eighty-ve per cent of they nanctance of thecoils, which would be required if the side circuits were loaded in theordinary way for ordinary voice frequency transmission. rThe phantomloading effect of the carrier circuit loading coils thus makes necessarythe use of different inductances in the phantom loading coils than arerequired in the ordinary voice loading of toll entrance cables connectedwith non-loaded lines.

Tt will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

l. Tn a transmission system comprising a phantomed group of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory trans mission for a range of frequencies higherthan that transmitted over the phantom and the phantom circuit of whichis loaded by coils spaced at greater intervals than the side circuits, aloading unit to be included in the phantomed group of conductors at thephantom loading points, said loading unit comprising a phantom loadingcoil and side circuit loading coils, the inductance of the side circuitloading coils being less than that of the side circuitl loading coils atnonphantom loading points by an amount equal to the leakage inductanceintroduced into the side circuit at the'phantom loading point by thephantom loading coil.

2. lin a transmission system comprising a phantomed group of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory transmission for a range of frequencies higherthan that transmitted over the phantom and the phantom circuit of whichis loaded by coils spaced at greater intervals than the side'circuits, aloading unit comprising a phantom loading coil and side circuit loadingcoils to be included in the phantomed group of conductors at a phantomloading point, said phantom loading coil introduclng leakage inductancein the side circuit coils and the side circuit coils of the unit beingdesigned' to add sucient inductance to the side circuits at the phantomloading oint over and above the leakage inductance of the phantom toobtain a total coil inductance value in the side circuit at the phan tomloading points equal to that supplied by the side circuit loadingcoils'installed atnon-phantom loading points.

3. A transmission system including a phantomed roup of conductors,loading coils inserte group at certain intervals, loading coils includedin the phantom circuit of said group at loading points coinciding withside circuit loading points and at intervals which are multiples of theside circuit loading intervals, the inductance of the side circuitloading coils inserted at phantom loading points being less than that ofthe side circuit coils at non-phantom loading points by an amount equalto the leakage inductance introduced into the side circuit by thephantom coil.

4c. ln a transmission system comprising av phantomed group ofconductors, the side circuits of which are loaded by coils spaced atintervals such as to give satisfactory transmission ior a range offrequencies higher than that transmitted over the phantom and thephantom circuit of which is loaded by coils spaced at greater intervalsthan the side circuits, a loading unit for use in said phantomed groupat phantom loading'points comprising a phantom loading coil and side inthe side circuits of said Leones@ Y 5. ln a transmission systemcomprising a l phantomed group of conductors, the side circuits of whichare loaded by coils spaced at intervals such as to give satisfactorytransmission for a range of frequencies higher than that transmittedover the phantom and the phantom circuit of which is loaded by coilsspaced at greater intervals than thel side circuits, a loading unit foruse at phantom loading points of said phantomed group, said loading unitcomprising a phantom coil and side circuit coils, said coils being sodesigned that the total mutual capacity introduced in the side circuitsby the loading unit is approximately the same as the mutual capacity ofthe loading coils which are used atl non-phantom loading points. Q

6.' ln a transmission system comprising a phantomed group of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory transmission for a range of frequencies higherthan that transmitted over the phantom and the phantom circuit of whichis loaded by coils spaced at greater intervals than the side circuits, aloading unit to be used at phantom loading points of said phantomedgroup, said loading unit comprising a phantom coil and side circuitcoils, the windings of the phantom coils being so arranged that thecapacity distribution oi the phantom coil with reference to the sidecircuit will be substantially the same as the capacity distribution ofthe side circuit coil with respect to the side circuit.

7. ln a transmission system comprising a phantomed group of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory transmission for a range of frequencies higherthan that transmitted vover the phantom and the phantom circuit of whichis loaded by coils spaced at greater intervals than the side circuits, aloading unit to be included at phantom loading points in said phantomedgroup, said loading unit comprising a phantom coil and side circuitcoils, the windings of the'phantomcoil being so arranged thatthecapacityybetween individual windings and the capacity between thewindings and the pot will be distributed with reference to the sidecircuit in the same manner as the corresponding capacities of the sidecircuit coil are distributed.

8. In a transmission system comprisin a phantomed group of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory transmission for a range of frequencies higherthan that transmitted over the phantom and the phantom circuit of whichis loaded by coils spaced at eater intervals than the side circuits, aloa ing unit for use at phantom loading oints of the phan- `tomed group,said loa ing unit comprising a phantom coil and side circuit coils, thephantom coil being made up of inner and outer windin s upon eachquadrant of a core and the side circuit coil including inner and outerwindings upon each half of the core, each side circuit conductor passingthrou h an inner Winding on one quadrant of a p antom core and anotherwinding upon another quardrant of a phantom core and also passingthrough an inner and outer winding upon different halves of the sidecircuit core, the winding connections of the hantom coil being suchthat, the capacity etween inner and outer windings and the capacitybetween the outer winding and the pot will be distributed with respectto the side circuit in the same manner as the corresponding capacities'of the side circuit coils and one of the windings upon each quadrant ofthe phantom coil bein wound in the opposite direction from t e otherwinding.

9. In atransmission system comprising a phantomed group of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory transmission for a range of frequencies higherthan that transmitted over the phantom and the phantom circuit ofwhichis loaded by coils spaced at greater intervals than the sidecircuits, a loading unit for use at phantom loading points of aphantomed group, said loading unit comprising a phantom coil and sidelcircuit coils, the windings of said side circuit coils being connectedin the side circuit at the electrical center of the phantom coil linewindings which are included in the side circuit conductors. l l

10. In a transmission system comprising a phantomedgroup of conductors,the side circuits of which are loaded by coils spaced at intervals suchas to give satisfactory transmission for a range of. frequencies higherthan that transmitted over the phan- .tom and the phantom circuit ofwhich is loaded by coilsspaced at greater intervals than the sidecircuits, a loading unit for use at phantom loading points of saidphantomed group, said loading unit comprising a phantom coil and sidecircuit coils, the Winding of the phantom coils being so arranged thatthe capacity of the coil with respect to the side circuit will bedistributed in a manner similar to that of the capacity of the sidecircuit, coil and the windings of the side circuit coils being connectedin the side circuit at the electrical center of the phantom coil linewindings included in the side circuit conductors.

, 11. In a transmission system comprising a phantomed roup ofconductors, the side circuits of W ich arel loaded by coils spaced atintervals such as to give satisfactory transmission for a range offrequencies higher than that transmitted over the phantom and thephantom circuit of which is loaded by coils spaced at reater intervalsthan the side circuits, a loa ing unit for use at phantom loadin pointsof said phantomed group, said p antom unit comprisin phantom coils andside circuit coils, sai coils being so designed as to obtainapproximately the same' total resistance in the side circuits at thephantom loading points as is introduced by 4side circuit coils includedat non-phantom loading points. p

In testimony whereof, we have signed our names to this specificationthis 29th day of June, 1922.

WLLIAM H. MARTIN. THOMAS SHAW.

